Semiconductor device and automotive AC generator

ABSTRACT

A semiconductor device includes a semiconductor element, a support member bonded to a first surface of the semiconductor element with a first bonding material, and a lead electrode bonded to a second surface of the semiconductor element supported on the support member with a second bonding material. Respective connecting parts of the support member and the lead electrode are Ni-plated and each of the first and the second bonding material is a Sn solder having a Cu 6 Sn 5  content greater than a eutectic content.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device fabricated byusing a solder capable of maintaining a reliable bonding ability at hightemperatures and an automotive ac generator (alternator) provided withthe semiconductor device.

2. Description of the Related Art

As mentioned in JP-A 07-221235 (patent document 1) by way of example, asemiconductor device for an automotive ac generator is constructed so asto reduce thermal stress that is induced therein due to difference inthermal expansion between the semiconductor device and electrodes sothat the semiconductor device can withstand a sever thermal cycle. Sincethe automotive ac generator is installed near an engine, thesemiconductor device included in the automotive ac generator is requiredto withstand a high temperature of 200° C. Therefore, the electrodes ofthe semiconductor device are soldered to circuit terminals with, forexample, a high-Pb solder having a solidus around 300° C., such as aPb—Sn alloy containing 95% by weight Pb and 5% by weight Sn and having asolidus of 300° C. and a liquidus of 314° C.

From a viewpoint of environmental protection, there is a demand forsemiconductor devices using a bonding material not containing Pb whichimparts a heavy load to the environment. An Au-20Sn solder (eutectic,280° C.) an Au-12Ge solder (eutectic, 356° C.) and an Au-15S solder(eutectic, 363° C.) are Pb-free solders not containing Pb and havingproperties similar to those of high-Pb solders. However, those Ausolders, namely, Pb-free solders, are very expensive. The Au-20Sn solderhaving a comparatively low Au content is a hard solder incapable ofsatisfactorily relaxing stress induced in a wire area and hence asemiconductor device having electrodes bonded to terminals by this hardsolder is likely to break.

A Sn solder, such as a Sn-3Ag-0.5Cu solder having a melting point notlower than 200° C., is another Pb-free solder having a medium meltingpoint. This Sn solder is used prevalently for mounting parts to a wiringboard and has satisfactory bond reliability at temperatures not higherthan 150° C. However, if parts soldered by this Sn solder are kept for along time in a working environment of 200° C. or above, an interfacialreaction occurs in the interface between the bonded parts. Consequently,voids are formed and intermetallic compounds grow and the bondreliability is reduced.

A method of suppressing the interfacial reaction of the Sn solderdisclosed in Jpn. Pat. No. 3152945 (patent document 2) uses a Sn soldercontaining 0.1 to 2% by weight Cu, 0.002 to 1% by weight Ni and Sn asthe remainder. It is mentioned in patent document 2 that Cu contained inthis Sn solder controls the Cu erosion of the materials of bonded partsand Ni contained in this Sn solder controls the growth of intermetalliccompounds, such as Cu₆Sn₅ and Cu₃Sn in interface between bonded parts. Amethod of forming a solder bump mentioned in JP-A 2002-280417 (patentdocument 3) forms two kinds of metal layers that interact with a Snsolder and form intermetallic compounds on the surface of a part to bebonded to another part and bonds a Sn solder ball to the surface. It ismentioned in patent document 3 that an interfacial reaction can besuppressed by thus forming a thin layer of an intermetallic compoundcontaining two or three elements including Sn in the interface betweenthe bonded parts.

SUMMARY OF THE INVENTION

Those known techniques have the following problems, are not satisfactoryin suppressing an interfacial reaction and have low bond reliability. Itis known that those known techniques are unable to suppress interfacereaction in the semiconductor device included in an automotive acgenerator (alternator) which is used in a high-temperature-environment.

The method disclosed in patent document 2 using the Sn solder containingNi is expected to suppress an interfacial reaction to some extent.However, an interfacial reaction occurs at high temperatures not lowerthan 200° C. because Cu₆Sn₅ and Cu₃Sn are always in contact with the Cuand the Sn solder. Consequently, grains of a Cu—Sn compound growcontinuously, voids are formed in the interface and the bond reliabilityis reduced.

The method of forming a solder bump disclosed in patent document 3 isexpected to have a high interfacial reaction suppressing effect becausethe intermetallic compound layer nearest to the solder bump serves as abarrier layer between the Sn solder and the metal layer and exercises ahigh interfacial reaction suppressing effect. However, two metal layers,namely, a first metal layer and a second metal layer, need to be formedpreviously on a part to be bonded. Therefore, an additional platingprocess for selective local plating is necessary, which increases thecost. It is difficult in some cases to form the metal layer whenelectrodes cannot be formed. The metal layer formed on the bondingsurface needs to react with the Sn solder when the metal layer is bondedto the Sn solder to form the barrier layer. If the metal layer is thick,an unreacted part remains in the surface metal layer and the effect ofthe barrier layer is unsatisfactory. Thus it is possible that theprocess needs to be adjusted to extend bonding time to make the metallayer react completely. If the surface metal layer is thin, the barrierlayer for suppressing an interfacial reaction is thin and the thinbarrier layer possibly cannot satisfactorily suppress an interfacialreaction at high temperatures not lower than 200° C. As shown in FIG. 2,exposed unreacted parts of the metal layer 15, such as a Cu layer,remaining after interaction between the Sn solder and the metal layer15, oxidation and corrosion starts from the exposed unreacted parts ofthe metal layer 15. As shown in FIG. 3, if a bonding surface outermostlayer is formed locally by local plating to prevent the survival of thebonding surface outermost layer, the Sn solder possibly spreads and weta metal layer 11, such as a Ni layer, underlying the bonding surfaceoutermost layer. Consequently, an intermetallic compound 16, such as aNi—Sn compound is formed, an interfacial reaction occurs in this partand voids are likely to be formed due to volume change.

Accordingly, it is an object of the present invention to provide asemiconductor device fabricated by using a low-cost bonding material forbonding a semiconductor device to a circuit, imparting low load to theenvironment and capable of maintaining bond reliability for a long timeof use at high temperatures not lower than 200° C. Another object of thepresent invention is to provide an automotive ac generator provided withthe semiconductor device.

A semiconductor device according to a first aspect of the presentinvention includes: a semiconductor element; a support member bonded toa first surface of the semiconductor element with a first bondingmaterial; and a lead electrode bonded to a second surface of thesemiconductor element supported on the support member with a secondbonding material; wherein respective connecting parts of the supportmember and the lead electrode are Ni-plated, each of the first and thesecond bonding-material is a Sn solder having a Cu₆Sn₅ content greaterthan a eutectic content.

A semiconductor device according to a second aspect of the presentinvention includes: a semiconductor element; a support member bonded toa first surface of the semiconductor element with a first bondingmaterial; and a lead electrode bonded to a second surface of thesemiconductor element supported on the support member with a secondbonding material; wherein respective connecting parts of the supportmember and the lead electrode are Ni-plated, each of the first and thesecond bonding material is a Sn solder containing Cu₆Sn₅ in atemperature range between a room temperature and 200° C.

A semiconductor device according to a third aspect of the presentinvention includes: a semiconductor element; a support member bonded toa first surface of the semiconductor element with a first bondingmaterial; and a lead electrode bonded to a second surface of thesemiconductor element supported on the support members with a secondbonding material; wherein a plated Ni layer and a Cu—Sn compound layerare formed in each of an interface between the support member and thefirst bonding material and an interface between the first bondingmaterial and the semiconductor element, and a plated Ni layer and aCu—Sn compound layer are formed in each of an interface between the leadelectrode and the second bonding material and an interface between thesecond bonding material and the semiconductor element.

An automotive ac generator according to the aspects of the presentinvention is provided with any one of the foregoing semiconductordevices.

The aspects of the present invention provides a semiconductor devicethat imparts a low load to the environment and capable of withstandinghigh-temperatures not lower than 200° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical sectional view of a bonded structure according tothe present invention;

FIG. 2 is a typical sectional view of a possible bonded structurementioned in patent document 3;

FIG. 3 is a typical sectional view of a possible bonded structurementioned in patent document 3;

FIG. 4 is a typical sectional view of a bonding mechanism according tothe present invention;

FIG. 5 is diagram showing the Young's moduli and yield strengths ofpossible buffering materials;

FIG. 6 is a typical sectional view of a bonding mechanism according toan embodiment of the present invention;

FIG. 7 is a typical sectional view of a bonding mechanism according toan embodiment of the present invention;

FIG. 8 is a typical sectional view of a semiconductor device in anembodiment according to the present invention;

FIG. 9 is a typical sectional view of a semiconductor device accordingto the present invention;

FIG. 10 is a typical view of voids formed in the interface betweenbonded surfaces;

FIG. 11 is a typical view of voids formed in the interface betweenbonded surfaces;

FIG. 12 is an SEM microphotograph of a section of a part of a bondedstructure around the interface between the bonded surfaces after ahigh-temperature endurance test;

FIG. 13 is an SEM microphotograph of a section of a part of a bondedstructure around the interface between the bonded surfaces after ahigh-temperature endurance test;

FIG. 14 is a phase diagram of a Cu—Sn alloy;

FIG. 15 is a typical view of a semiconductor device in an embodimentaccording to the present invention;

FIG. 16 is a typical view of a semiconductor device in, an embodimentaccording to the present invention; and

FIG. 17 is a typical view of a semiconductor device in an embodimentaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A bonding material and a bonding mechanism applied to a semiconductordevice in a preferred embodiment according to the present invention willbe described with reference to FIG. 4.

A bonding material to be used for fabricating the semiconductor deviceaccording to the present invention is a Sn solder foil 17 made of a Snsolder containing Cu—Sn compound grains 10, such as Cu₆Sn₅ grains 10, attemperatures between a room temperature and 200° C. When parts 12 eachhaving a surface coated with a Ni layer 11 formed by plating are bondedby the Sn solder foil 17, the Cu₆Sn₅ grains 10 contained in the Snsolder foil 17 deposit on or migrate to the plated Ni layers 11 andcompound layers 10 containing a Cu—Sn compound as a principal componentare formed. The compound layer 10 each formed between the plated Nilayer and the Sn solder serve as barrier layers. The barrier layerssuppress the growth of compound layers Resulting from an interfacialreaction and the formation of voids resulting from the growth ofcompound layers at high temperatures not lower than 200° C.

A bonding mechanism applied to the semiconductor device according to thepresent invention includes at least one plated layer of Ni, Ni—P or Ni—Bformed on a member to be bonded. Therefore, bonding can be achieved by asmall number of steps. The thickness of the barrier layer included inthe bonding mechanism according to the present invention is dependent onthe Cu—Sn compound content of the Sn solder foil. Therefore, thethickness of the barrier layer can be adjusted by adjusting the Cu—Sncompound content of the Sn solder foil. As shown in FIG. 1, the Cu—Sncompound grains 10 contained in the Sn solder deposit on or migrate tothe plated Ni layer 11 actively in the interface wetted with the Snsolder and form a barrier layer of the Cu—Sn compound. Therefore, theforegoing problems shown in FIGS. 2 and 3 do not arise in the joint.

Conditions that the bonding material contains Cu—Sn compound grains andthe bonding material is a Sn solder containing Cu₆Sn₅ at temperaturesbetween a room temperature and 200° C. will, be described with referenceto a phase diagram shown in FIG. 14.

When a molten solder of a composition having a Cu content smaller thanSn-0.9Cu solidifies, Sn contained in the solder in a Sn content greaterthan a eutectic content deposit in primary crystals in an initial stageof solidification, and eutectic grains of Sn and Cu₆Sn₅solidify in afinal stage of solidification. When the solder solidifies, Cu₆Sn₅ grainsare dispersed in grain boundaries in the joint and do not form a barrierlayer on the plated Ni layer. Therefore, heat resistance isunsatisfactory. When a molten solder of a composition having a Cucontent greater than Sn-0.9Cu solidifies, Cu₆Sn₅ deposits first. SinceCu₆Sn₅ deposits preferentially on the plated Ni layer and a barrierlayer of a Cu—Sn compound is formed. A eutectic of Sn and Cu₆Sn₅solidifies in a final stage of solidification. Thus the barrier layer ofthe Cu—Sn compound is formed.

The desirable bonding material for fabricating the semiconductor deviceaccording to the present invention has a composition having a Cu₆Sn₅content greater than a eutectic content. A desirable Cu content of aSn—Cu two-element bonding material is 0.9% by weight or above. Alloyscontaining elements in addition to Cu and Sn have different eutecticcompositions, respectively. Therefore, a desirable bonding material hasa Cu₆Sn₅ content greater than a eutectic content. Each of a Sn-3Ag-0.5Cumaterial and a Sn-0.7Cu material, which are generally used, having aCu₆Sn₅ content smaller than a eutectic content does not form a barrierlayer on the plated Ni layer.

The bonding material does not need to be supplied in a foil and may besupplied in a paste as shown in FIG. 6 or in a wire as shown in FIG. 7.Either of the paste of the bonding material and the wire of the bondingmaterial forms a barrier layer of a Cu—Sn compound on the plated Nilayer. The bonding material may be supplied by a supplying methodsuitable for the bonding environment.

It is preferable for satisfactory wettability that the Sn soldercontaining Cu₆Sn₅ at temperatures between, a room temperature and 200°C. has a composition having a liquidus of a temperature not higher thana bonding temperature.

The semiconductor device in the preferred embodiment for an automotiveac generator and a method of fabricating the same using the bondingmaterial meeting the requirements of the present invention will bedescribed with reference to FIGS. 8 and 9.

Referring to FIG. 8, the semiconductor device includes a semiconductorelement 1, a lead electrode 7 having a Ni-plated connecting part bondedto a first surface of the semiconductor element 1 by a bonding layer 2of the bonding material of the present invention, a buffer member 5, forabsorbing stress resulting from thermal expansion difference, having aNi-plated connecting part and bonded to a second surface of thesemiconductor element 1 by a bonding layer 4 of the bonding material ofthe present invention, and a support member 3 bonded to the buffermember 5 by a bonding layer 6 of the bonding material of the presentinvention.

Since the components of the semiconductor device are bonded with thebonding material meeting the requirements of the present invention, aninterfacial reaction in the semiconductor device can be suppressed andthe semiconductor device has bond reliability when the semiconductordevice is used in a high-temperature environment. Although all thebonding layers do not necessarily need to be made of the bondingmaterial meeting the requirements of the present invention and some ofthe bonding layers may be made of a bonding material other than thebonding material meeting the requirements of the present invention, itis preferable that all the bonding layers are made of the bondingmaterial meeting the requirements of the present invention from the viewpoint of ensuring bond reliability. The bonding material may be of anycomposition, provided that the Cu₆Sn₅ content is greater than theeutectic content. The bonding layers may be made of different materials,respectively.

The buffer member 5 may be made of any one of Al, Mg, Ag, Zn, Cu and Ni.Each of those metals has a low yield strength and members made of thosemetals are easily deformable. The buffer member 5 absorbs a stressinduced in the bonded parts due to difference in thermal expansionbetween the bonded parts when the bonded parts are cooled and aresubjected to a thermal cycle test. As shown in FIG. 5, it is desirablethat the yield strength is 75 MPa or below. If the yield strength is 100MPa or above, the buffering effect of the buffer member 5: isinsufficient and, in some cases, the semiconductor element 1 cracks.Preferably, the buffer member 5 has a thickness between 30 and 500 μm.If the thickness of the buffer member 5 is below 30 μm, the buffermember 5 is unable to absorb stress sufficiently and, in some cases, thesemiconductor element 1 and the intermetallic compound-layers crack. Ifthe thickness of the buffer member 5 is greater than 500 μm, the buffermember 5 the effect of the thermal expansion of the buffer member 5exceeds the buffering effect of the buffer member 5 because Al, Mg, Agand Zn have coefficients of thermal expansion greater than the Cuelectrode, and the buffer member 5 having such a big thickness reducesreliability.

The buffer member 5 may be made of a Cu/Invar/Cu composite material, aCu/Cu20 composite material, a Cu—Mo alloy, Ti, Mo or W. The buffermember 5 can absorb stress induced in the bonding layer due todifference in coefficient of thermal expansion between the semiconductorelement and the Cu electrode when the semiconductor device is subjectedto thermal cycles and when the semiconductor device is cooled after thebonding processes. If the buffer member 5 is excessively thin, thebuffer member 5 cannot satisfactorily absorb stress and, in some cases,the semiconductor element and the intermetallic compound layers crack.Preferably, the thickness of the buffer member 5 is 30 μm or above.

The Sn solder has a thermal conductivity higher than those of high-leadsolders. Therefore, the use of the Sn solder is effective in reducingthe resistance of the semiconductor device and heat radiation from thesemiconductor device. Although the buffer member 5 may be omitted asshown in FIG. 9, it is preferable that the semiconductor device isprovided with the buffer member 5 for satisfactory bond reliability evenif the Sn solder harder than the high-lead solder is used.

The Ni layers formed by plating on the parts may be Ni layers, Ni—Players or Ni—B layers. The Ni layers may be coated with coated with anAu layer or an Ag layer to improve solder-wettability. The Au layer orthe Ag layer can be diffused entirely in the solder during bonding andhence the Cu—Sn compound barrier can be formed on the Ni layer.

A method of fabricating the semiconductor device shown in FIG. 8 will bedescribed. The Sn solder foil containing Cu₆Sn₅ at temperatures betweena room temperature and 200° C., namely, a bonding member 6, theNi-plated buffer member 5 of 6.8 mm in diameter and 0.6 mm in thickness,made of a cladding material (Cu/Invar/Cu) having a coefficient ofthermal expansion of 11×10⁻⁶/° C., the Sn solder foil 4 containingCu₆Sn₅ at temperatures between a room temperature and 200° C., theNi-plated semiconductor element 1 of 6 mm in diameter and 0.2 mm inthickness, the Sn solder foil 2 containing Cu₆Sn₅ at temperaturesbetween a room temperature and 200° C., and the lead electrode 7 havinga Cu plate of 4.5 mm in diameter and 0.2 mm in thickness are stacked upin that order on the support member 3 to form a layered structure. Thelayered structure is subjected to a bonding process to heat the layeredstructure at 380° C. for 1 min in a reducing nitrogen atmospherecontaining 50% hydrogen to bond the adjacent components by the Snsolder. Then, silicone rubber 8 is poured into a space in the supportmember 3 and the silicone rubber is cured to complete the semiconductordevice. When the bonding process is heats the layered structure at atemperature between 220° C. and 450° C. in a reducing atmosphere, theadjacent components can be satisfactorily bonded together without usingany flux.

Semiconductor devices in Examples 1 to 12 thus fabricated and those inComparative examples 1 to 5 were subjected to a thermal cycle test and ahigh-temperature endurance test, and bond strengths holding the adjacentcomponent parts of the semiconductor devices were measured after thosetests.

Measured data is shown in Table 1. TABLE 1 Composition of the solderThermal cycle test High-temperature endurance (percent by mass) Bondedmember Construction (500 cycles) test (1000 h) Examples 1 Sn—3Cu Niplated ◯ ◯ 2 Sn—5Cu Ni plated ◯ ◯ 3 Sn—7Cu Ni plated ◯ ◯ 4 Sn—3Ag—3Cu Niplated ◯ ◯ 5 Sn—3Ag—5Cu Ni plated ◯ ◯ 6 Sn—5Cu Ni—P plated ◯ ◯ 7 Sn—3CuNi plated ◯ ◯ 8 Sn—5Cu Ni plated ◯ ◯ 9 Sn—7Cu Ni plated ◯ ◯ 10Sn—3Ag—3Cu Ni plated ◯ ◯ 11 Sn—3Ag—5Cu Ni plated ◯ ◯ 12 Sn—5Cu Ni—Pplated ◯ ◯ Comparative examples 1 Sn—2Ag Ni plated ◯ X 2 Sn—3Ag—0.5Cu Niplated ◯ X 3 Sn—2Ag Cu X X 4 Sn—3Ag—0.5Cu Cu X X 5 Pb—10Sn Ni plated ◯ ◯

In Table 1, circles indicate the semiconductor devices in which bondstrength after the test was not lower than 80% of an initial bondstrength and crosses indicate the semiconductor devices in which bondstrength after the test was below 80% of an initial bond strength. Thethermal cycle included the steps of cooling at −40° C. for 30 min andheating at 200° C. for 30 min. The thermal cycle was repeated 500 timesfor the thermal cycle test. The high-temperature endurance test kept thesemiconductor devices at 210° C. for 1000 h. Bond strengths after thehigh-temperature endurance test in the semiconductor devices in Examples1 to 6 were not lower than 80% of the bond strengths before thehigh-temperature endurance test. Changes in thermal resistances of thesemiconductor devices in Examples 1 to 6 caused by the test were notgreater than 10%. FIG. 12 shows a section of a test piece formed bybonding parts with a Sn-5Cu solder and kept at 210° C. for 1100 h. Asobvious from FIG. 12, a barrier layer of a Cu—Sn compound prevented theloss of an Ni layer due to heating at a high temperature and any voidsdue to volume change were not formed.

Although the construction of the semiconductor device and the bondingprocess of simultaneously bonding the component parts with the Sn soldercontaining Cu₆Sn₅ at temperatures between a room temperature and 200°C., the component parts may be sequentially bonded together. Forexample, the buffer member 5, the semiconductor element 1 and the leadelectrode 3 provided with the Cu plate may be bonded together with theSn solder containing Cu₆Sn₅ at temperatures between a room temperatureand 200° C. to form a subassembly, and then the subassembly may bebonded to the support member 3 with the Sn solder containing Cu₆Sn₅ attemperatures between a room-temperature and 200° C.

Referring to FIG. 9, the Sn solder foil 4 containing Cu₆Sn₅ attemperatures between a room temperature and 200° C., namely, a bondingmember, the Ni-plated semiconductor element 1 of 6 mm in diameter and0.2 mm in thickness, the Sn solder foil 2 containing Cu₆Sn₅ attemperatures between a room temperature and 200° C., and the leadelectrode 7 having a Cu plate of 4.5 mm in diameter and 0.2 mm inthickness are stacked up in that order on the support member 3 to form alayered structure. The layered structure is placed in a positioning jig.The layered structure is subjected to a bonding process to heat thelayered structure at 450° C. for 5 min in a reducing nitrogen atmospherecontaining 50% hydrogen to bond the adjacent components by the Snsolder. Then, silicone rubber 8 is poured into a space in the supportmember 3 and the silicone rubber is cured to complete the semiconductordevice.

Semiconductor devices in Examples 1 to 12 thus fabricated and those inComparative examples 7 to 12-were subjected to a thermal cycle test, anda high-temperature endurance test, and bond strengths holding theadjacent component parts of the semiconductor devices were measuredafter those tests. Measured data is shown in Table 1. In Table 1,circles indicate the semiconductor devices in which bond strength afterthe test was not lower than 80% of an initial bond strength and crossesindicate the semiconductor devices in which bond strength after the testwas below 80% of an initial bond strength. The thermal cycle includedcooling at −40° C. for 30 min and heating at 200° C. for 30 min. Thethermal cycle was repeated 500 times for the thermal cycle test. Bondstrengths after the thermal cycle test in all the semiconductor devicesin Examples 7 to 12 were not lower than 80% of the bond strengths beforethe thermal cycle test. Bond strengths after the high-temperatureendurance test in all the semiconductor devices in Examples 7 to 12 werenot lower than 80% of the bond strengths before the high-temperatureendurance test. The high-temperature endurance test kept thesemiconductor devices at 210° C. for 1000 h.

Description will be made of measured data on semiconductor devices incomparative examples fabricated by using a bonding material of acomposition having a Cu₆Sn₅ content lower than a eutectic content.

The semiconductor devices in Comparative examples 1 to 5 are the same inconstruction as the semiconductor devices in Examples 1 to 6. Measureddata is shown in Table 1. In Table. 1, circles indicate thesemiconductor devices in which bond strength after the test was notlower than 80% of an initial bond strength and crosses indicate thesemiconductor devices in which bond strength after the test was below80% of an initial bond strength. The thermal cycle included the steps ofcooling at −40° C. for 30 min and heating at 200° C. for 30 min. Thethermal cycle was repeated 500 times for the thermal cycle test. Thehigh-temperature endurance test kept the semiconductor devices at 210°C. for 1000 h. Bond strengths after the thermal cycle test in thesemiconductor devices in Comparative examples 1 and 2 were not lowerthan 80% of the bond strengths before the thermal cycle test. Bondstrengths after the high-temperature endurance test in all thesemiconductor devices in Comparative examples 1 and 2 were below 80% ofthe bond strengths before the high-temperature endurance test. Voids 14as shown in FIGS. 10 and 11 were formed in the interface between thebonded component parts. It is inferred that an interfacial reaction waspromoted when the semiconductor device is kept at 210° C. for 1000 h,voids were formed due to volume change resulting from the growth of acompound layer and the voids reduced the bond strength. FIG. 13 shows asection of a structure formed by bonding parts with a Sn-3Ag-0.5Cusolder and heated at 210° C. for 1000 h. Since any barrier layer of aCu—Sn compound is not formed, Sn and Ni interacted and a Ni layerdisappeared completely and a Cu layer reacted with Sn and a thick Cu—Sncompound layer is formed. Voids formed due to a big volume changedeteriorate the condition of bonding.

Bond strengths after the thermal cycle-test, in which a thermal cycleincluding the steps of cooling at −40° C. for 30 min and heating at 200°C. for 30 min was repeated 500 times, in the semiconductor devices inComparative examples 3 and 4 were below 80% of the bond strengths beforethe thermal cycle test. Bond strengths after the high-temperatureendurance test in all the semiconductor devices in Comparative examples3 and 4, similarly to those of the semiconductor devices in Comparativeexamples 1 and 2, were below 80% of the bond strengths before thehigh-temperature endurance test. Voids 14 as shown in FIGS. 11 and 11were formed in the interface between the bonded component parts. It isinferred that an interfacial reaction was promoted when thesemiconductor device was kept at a high temperature, voids were formeddue to volume change resulting from the growth of a compound layer andthe voids reduced the bond strength.

A semiconductor device in Comparative example 5 was fabricated by usinga high-lead solder and had satisfactory properties.

A semiconductor device in a second embodiment according to the presentinvention fabricated by using the bonding material of the presentinvention will be described with reference to FIG. 15. The semiconductordevice shown in FIG. 15 includes a printed wiring board 102, asurface-mounted package 101 bonded to the printed wiring board 102 withthe bonding material of the present invention, a package 103 bonded tothe printed wiring board 102 with the bonding material of the presentinvention and a through hole package 104 connected to the printed wiringboard 102 with the bonding material of the present invention. Surfacesof parts, not shown, of the packages mounted on the printed wiring board102 are Ni-plated. The bonding material of the present inventionsuppresses an interfacial reaction even at high temperatures. Therefore,the semiconductor device has high bond reliability.

The semiconductor device shown in FIG. 15 is provided with thesurface-mounted package 101, the package 103 and the through holepackage 104. The semiconductor device may be provided with one or two ofthose three packages. A solder other than the bonding material of thepresent invention, such as a Sn-3Ag-0.5Cu solder, may be used forbonding some parts of the packages to the printed wiring board 102.

A semiconductor device in a third embodiment according to the presentinvention fabricated by using the bonding material of the presentinvention will be described with reference to FIG. 16.

The semiconductor device shown in FIG. 16 includes a semiconductorelement 1, a frame 105 bonded to the semiconductor element 1 with thebonding-material of the present invention, an external lead 107electrically connected to an electrode, not shown, of the semiconductorelement 1 by a wire 108, and a molded resin package 106 covering thesemiconductor element 1. Surfaces, not shown, to be bonded to otherparts are Ni-plated. The bonding material of the present inventionsuppresses an interfacial reaction even at high temperatures and thesemiconductor device has high bond reliability.

A semiconductor device in a fourth embodiment according to the presentinvention will be described with reference to FIG. 17.

The semiconductor device shown in FIG. 17 has a configurationrepresented by that of a RF module. The semiconductor device includes amodule substrate 109, a surface-mounted package 101 bonded to the modulesubstrate with the bonding material of the present invention, asemiconductor element 1 bonded to the module substrate with the bondingmaterial of the present invention, a package bonded to the modulesubstrate with the bonding material of the present invention, and solderballs 110 attached to the back surface of the module substrate 109.Surfaces, not shown, to be bonded to other parts are Ni-plated. Thebonding material of the present invention suppresses an interfacialreaction even at high temperatures and the semiconductor device has highbond reliability.

The semiconductor device shown in FIG. 15 is provided with thesurface-mounted package 101, the package 103 and the through holepackage 104. The semiconductor device may be provided with one or two ofthose three packages. A solder other than the bonding material of thepresent invention, such as a Sn-3Ag-0.5Cu solder, may be used forbonding some parts of the packages to the printed wiring board 102.

The present invention is not limited to the foregoing embodiments. Forexample, the present invention may be applied to die bonding in formingfront end modules, such as power transistors, power ICs, IGBT substratesand RF modules, and automotive power modules. The bonding material ofthe present invention may be used in any modes including a process ofleveling a printed wiring board, a process of coating parts by dippingand printing and may be used in any articles including foils and wires,provided that the bonding material has a composition having a Cu₆Sn₅content greater than a eutectic content.

The present invention provides a semiconductor device that imparts a lowload to the environment and capable of withstanding high temperaturesnot lower than 200° C.

Although the invention has been described in its preferred embodimentswith a certain degree of particularity, obviously many changes andvariations are possible therein. It is therefore to be understood thatthe present invention may be practiced otherwise than as specificallydescribed herein without departing from the scope and spirit thereof.

1. A semiconductor device comprising: a semiconductor element; a supportmember bonded to a first surface of the semiconductor element with afirst bonding material; and a lead electrode bonded to a second surfaceof the semiconductor element supported on the support member with asecond bonding material; wherein respective connecting parts of thesupport member and the lead electrode are Ni-plated and each of thefirst and the second bonding material is a Sn solder having a Cu₆Sn₅content greater than a eutectic content.
 2. The semiconductor deviceaccording to claim 1, wherein each of the first and the second bondingmaterial is a Sn solder containing Cu₆Sn₅ at temperatures between aroom-temperature and 200° C.
 3. A semiconductor device comprising: asemiconductor element; a support member bonded to a first surface of thesemiconductor element with a first bonding member; and a lead electrodebonded to a second surface of the semiconductor element supported on thesupport member with a second bonding member; wherein a plated Ni layerand a Cu—Sn compound layer are formed in each of an interface betweenthe support member and the first bonding member and an interface betweenthe first bonding member and the semiconductor elements and a plated Nilayer and a Cu—Sn compound layer are formed in each of an interfacebetween the lead electrode and the second bonding member and aninterface between the second bonding member and the semiconductorelement.
 4. The semiconductor device according to claim 1, wherein abuffer layer of a buffer material for absorbing a thermal expansiondifference between the support member and the semiconductor element isformed between the support member and the semiconductor element.
 5. Thesemiconductor device according to claim 4, wherein the buffer materialis Al, Mg, Ag, Zn, Cu or Ni.
 6. The semiconductor device according toclaim 4, wherein the buffer material is a Cu/Invar/Cu compositematerial, a Cu/Cu₂O composite material, a Cu—Mo alloy, Ti, Mo or W. 7.The semiconductor device according to claim 1, wherein the plated Nilayer is made of Ni, Ni—P or Ni—B.
 8. The semiconductor device accordingto claim 7, wherein at least one of a plated Au layer, a plated Ag layerand a plated Pd layer is formed on the plated Ni layer.
 9. Thesemiconductor device according to claim 1, wherein the Cu—Sn compound isCu₆Sn₅.
 10. The semiconductor device according to claim 1, wherein thebonding material is a foil, a paste or a wire.
 11. An automotive acgenerator provided with the semiconductor device according to claim 1.12. A semiconductor device comprising: a substrate; and a semiconductorelement bonded to the substrate with a bonding member; wherein a platedNi layer and a Cu—Sn compound layer are formed in each of an interfacebetween the substrate and the bonding member and an interface betweenthe bonding member and the semiconductor element.
 13. A semiconductordevice comprising: a substrate; and a semiconductor element bonded tothe substrate with a bonding material; wherein plated Ni layers areformed on a connecting surface of the substrate and a connecting surfaceof the semiconductor element, respectively, the bonding material is a Snsolder of a composition having a Cu₆Sn₅ content greater than a eutecticcontent.
 14. The semiconductor device according to claim 13, wherein theCu—Sn compound is Cu₆Sn₅.
 15. The semiconductor device according toclaim 13, wherein the bonding material is a foil, a paste or a wire.